1. Field of the Invention
The present invention relates to a ceramic circuit board comprising both high heat-cycle characteristic and high bonding strength characteristic that are suitable for a high-power transistor module circuit board, and more particularly, to a ceramic circuit board to which high thermal conductivity and high strength are imparted by bonding a metal circuit plate such as aluminum circuit plate to ceramic substrate such as silicon nitride substrate.
2. Description of the Related Art
Conventionally, a ceramic circuit board formed by bonding a metal circuit plate to a ceramic substrate has been widely applied to various electronic parts and machinery parts. Further, as a substrate for mounting electronic parts such as semiconductor chip or the like, various substrates such as ceramic substrate and resin substrate or the like have been practically used. Among these substrates, the ceramic substrate has been mainly used as the substrate for mounting a highly-heat radiating electronic part, because the ceramic substrate has an excellent insulating property and high-heat radiating property or the like.
Up to now, as the ceramic substrate, alumina sintered body has been mainly used because the alumina sintered body has been easily available. However, in accordance with an increase of a degree of integration, frequency, output power or the like of the semiconductor chip in these days, there is a tendency that an amount of heat to be generated from the semiconductor chip has remarkably increased. With respect to the tendency, the alumina substrate cannot meet the requirement for heat-radiating property. To cope with the above problem, there has been already proposed and practically used a ceramic substrate composed of aluminum nitride (AlN) sintered body having a thermal conductivity about ten-times larger than that of alumina and having a thermal expansion coefficient similar to that of Si.
That is, a circuit board having a high thermal conductivity has been widely used as parts for constituting the various electronic devices. For example, there has been widely used a ceramic circuit board in which aluminum nitride (AlN) sintered body having a high thermal conductivity of about 170 W/mK-class is used as the ceramic substrate or silicon nitride (Si3N4) sintered body having a high thermal conductivity of about 70 W/mK-class is used as the ceramic substrate.
The aluminum nitride substrate has above characteristics, however, mechanical strength and toughness of the AlN substrate are relatively small. Therefore, when the AlN substrate is tightly fastened by a screw at an assembling process of the devices using the circuit board, or when a heat cycle is applied to the AlN substrate, there may be posed a disadvantage that a crack is liable to occur. In particular, when the substrate is applied to power transistor modules for automobile, aircraft, machine tool, and robot or the like that are used and operated under severe load conditions and heat conditions, the above disadvantage would be further remarkable.
Therefore, as the ceramic substrate for mounting the electronic parts, it has been required to improve a mechanical reliability of the ceramic substrate. In this connection, there has been paid attention to a ceramic substrate composed of silicon nitride (Si3N4) sintered body having a thermal expansion coefficient similar to Si and excellences in mechanical strength and toughness, though the silicon nitride substrate is inferior to aluminum nitride substrate in thermal conductivity. In also the silicon nitride substrate, when grain size of silicon nitride material powder to be formed into sintered body and a composition of sintering agent in the material powder are appropriately controlled, a high thermal conductivity of, for example, 50 W/mK or more has been realized.
The ceramic circuit board in which silicon nitride (Si3N4) sintered body is used as the ceramic substrate can be manufactured in accordance with, for example, an active metal brazing method described hereunder.
At first, Agxe2x80x94Cuxe2x80x94Ti group brazing material is screen-printed on a surface of silicon nitride (Si3N4) substrate, then a metal circuit plate composed of Cu is disposed on the printed surface, thereafter, a heat treatment is carried out at a temperature of about 850xc2x0 C., so that the ceramic substrate and the metal circuit plate are bonded thereby to manufacture a ceramic circuit board.
In thus obtained ceramic circuit board, Ti as the active metal and N contained in the nitride type ceramic substrate are covalent-bonded to form TiN (titanium nitride), so that a bonding layer is formed by this TiN, whereby a high-bonding strength can be obtained to some extent.
However, in a case where the ceramic circuit board is applied to semiconductor module or the like to be equipped on vehicles, a severe heat-load is applied to the ceramic circuit board, so that fine cracks are disadvantageously occurred at peripheral portion of the metal circuit plate. In a case where the heat-cycle is further continued to be applied to the ceramic circuit board while remaining the fine cracks as they are, the metal circuit plate is peeled off from the ceramic substrate, so that there is posed a problem of inviting defectives in bonding strength and heat-resistance thereby to lower an operating reliability of the circuit board as an electronic device.
By the way, in the conventional ceramic circuit board formed by bonding the metal circuit plate to the ceramic substrate, a copper circuit plate has been mainly used as the metal circuit plate. This is because a copper has a high electric conductivity and low-distortion characteristic capable of obtaining an excellent circuit function.
However, in case of the ceramic circuit board where the silicon nitride substrate is used as the ceramic substrate and the copper circuit plate is used as the metal circuit plate, there can be provided a notable effect of preventing the crack formation caused by the fastening operation performed in the assembling process of the circuit board. In contrast, the effect of suppressing the crack formation to be caused by the fastening operation performed in the assembling process is still insufficient, so that there is a strong demand for the circuit plate to further improve the suppressing effect.
In addition, in recent years, a miniaturization of the electronic devices has been further advanced. In accordance with this advancement, it has been also strongly demanded to further reduce a mounting space of the circuit board. In this regard, since the conventional ceramic circuit board having a single-layered structure requires a predetermined plain surface area for being mounted to the devices or the like, so that there is a limit for reducing the mounting area.
The present invention has been achieved for solving the aforementioned problems. Accordingly, an object of the present invention is to provide a ceramic circuit board having a high bonding strength and a high-heat cycle resistance and capable of improving operating reliability as an electronic device.
Another object of the present invention is to provide a ceramic circuit board in which a silicon nitride substrate is used as a ceramic substrate, the ceramic circuit board being capable of exhibiting a remarkable effect of preventing crack formation to be caused by fastening operation performed in assembling process of the ceramic circuit board into devices, capable of greatly improve an effect of suppressing the crack formation to be caused at the silicon nitride substrate when heat cycle is applied to the ceramic circuit board in comparison with a case where a copper circuit plate is used as the metal circuit plate, whereby an availability and usability particularly in durability of the ceramic circuit board can be increased.
Still another object of the present invention is to provide a ceramic circuit board capable of contributing to reduce a mounting space for the ceramic circuit board to be mounted on a device without impairing the effect of suppressing crack formation caused by the heat cycle applied to the silicon nitride substrate.
In order to attain the objects described above, the inventors of the present invention had reviewed about various factors such as kind of a brazing material for bonding the metal circuit plate to the ceramic substrate, arrangement of a bonding layer composed of the brazing material or the like. As a result, the inventors had obtained the following findings.
The brazing material mainly composed of Al and constituting the bonding layer has a function of imparting wettability to ceramic substrates such as AlN and Si3N4 or the like, and firmly combines with the ceramic substrate thereby to metallize the ceramic substrate. Therefore, when the Al plate is applied as the metal circuit plate, it is preferable to bond the Al circuit plate to the ceramic substrate by using the brazing material mainly composed of Al.
On the other hand, when a copper plate is bonded to a ceramic substrate through the brazing material mainly composed of Al, Cu and Al are disadvantageously alloyed, so that the wettability of Al with respect to the ceramic substrate is lowered and the bonding operation becomes impossible. However, when an insulating layer composed of metal foil comprising at least one element selected from the group consisting of Ib, IIb, IIIb, IVb family elements such as Sn, In or the like prescribed in periodic table is disposed between the bonding layer and the copper plate, the insulating layer can effectively prevent Cu and Al from being alloyed to each other. As a result, it was confirmed that the ceramic substrate and the copper plate can be bonded with a high bonding strength.
Further, Sn as Ib family element and Al contained in the brazing material used for forming the insulating layer are soft metals each having an yield strength of 20 Mpa or less, so that a residual stress caused by a difference in coefficients of linear-thermal expansion between the copper plate and the ceramic substrate of the ceramic circuit board, and a thermal stress caused by the subsequent heat-load cycles applied to the circuit board can be remarkably reduced, so that a heat-cycle resistance is improved and the ceramic per se hardly cause crack due to the thermal stress. In addition, it was also confirmed that since the crack due to the heat cycle is hardly occurred, so that a bonding strength was not lowered at all.
The present invention had been achieved on the basis of the aforementioned findings. That is, according to a first aspect of the present invention, there is provided a ceramic circuit board comprising: a ceramic substrate and a metal circuit plate bonded to the ceramic substrate through a brazing material layer; wherein the brazing material layer is composed of Alxe2x80x94Si group brazing material and an amount of Si contained in the brazing material layer is 7 wt % or less.
In addition, in the ceramic circuit board according to the above invention, it is preferable that the ceramic substrate is composed of silicon nitride. Further, it is also preferable that the metal circuit plate is aluminum plate or aluminum alloy plate. Furthermore, it is also preferable that the aluminum alloy is Alxe2x80x94Si alloy. Further, it is also preferable that the metal circuit plate has a peel strength of 7 kg/cm or more.
In addition, a brazing material layer composed of Al and alkaline earth metal element or compound thereof can be also applied in place of Alxe2x80x94Si alloy. In this case, an amount of the alkaline earth metal element or compound thereof contained in the brazing material layer is preferably set to 12 wt % or less.
According to a second aspect of the present invention, there is provided a ceramic circuit board in which a metal circuit plate is bonded to a ceramic substrate, the ceramic circuit board comprising: a ceramic substrate composed of at least one of aluminum nitride (AlN), silicon nitride (Si3N4) and aluminum oxide (Al2O3); a bonding layer which is integrally formed to the ceramic substrate and consists of a brazing material mainly composed of Al; an insulating layer for preventing the brazing material from being alloyed with the metal circuit plate, the insulating layer being integrally formed to the bonding layer; and a metal circuit plate bonded to the ceramic substrate through the insulating layer.
In addition, in the ceramic circuit board according to the above second aspect of the present invention, it is preferable that the insulating layer is composed of a metal paste containing metal powder having a grain size of 10 xcexcm or less, or composed of a metal foil having a thickness of 10 xcexcm or more. Further, it is preferable that the insulating layer has a thickness of 10 xcexcm or more. Furthermore, it is preferable that the metal constituting the metal paste or the metal foil in the insulating layer is at least one metal of Ni, Sn and In.
According to a third aspect of the present invention, there is provided a ceramic circuit board in which a metal circuit plate is bonded to a ceramic substrate, the ceramic circuit board comprising: a ceramic substrate composed of at least one of aluminum nitride (AlN), silicon nitride (Si3N4) and aluminum oxide (Al2O3); a first bonding layer which is integrally formed to the ceramic substrate and consists of a brazing material or alloy mainly composed of Al; an insulating layer for preventing the first bonding layer from being alloyed with the metal circuit plate, the insulating layer being integrally formed to the first bonding layer; a second bonding layer which is integrally formed to a surface of the insulating layer and consists of a brazing material or alloy mainly composed of Al; and a metal circuit plate bonded to the ceramic substrate through the second bonding layer.
In the ceramic circuit board according to the third aspect of the present invention, it is preferable that each of the bonding layers and the insulating layer has a thickness of 10 xcexcm or more, respectively.
In order to achieve the aforementioned another object, according to a fourth aspect of the present invention, there is provided a ceramic circuit board comprising: a ceramic substrate; and a metal circuit plate bonded to the ceramic substrate; wherein the ceramic substrate is composed of a silicon nitride (Si3N4) substrate having a thermal conductivity of 50 W/mK or more and the metal circuit plate is composed of aluminum circuit plate, and wherein the silicon nitride substrate and the aluminum circuit plate are directly bonded without providing a brazing material layer. In this regard, as the aluminum circuit plate, single substance of Al or Al alloy is applied.
According to the fourth aspect of the present invention, since the metal circuit plate is composed of aluminum circuit plate in place of copper circuit plate, it becomes possible to greatly improve an effect of suppressing the crack formation in comparison with a case where the copper circuit plate is used, the crack being caused at silicon nitride substrate when the circuit board is used under a condition of being applied with the heat cycle.
That is, according to reviews and studies conducted by the inventors of the present invention, when copper (Cu) is applied with heat cycle of a predetermined temperature range, for example, within a range of 0xc2x0 C.-100xc2x0 C., the copper repeats heat-expansion and heat-contraction by causing elastic deformation thereby to maintain a constant shape (flat pattern in case of flat plate). This means for the ceramic circuit board formed by bonding the copper circuit plate to the silicon nitride substrate that the expansion and contraction caused by the heat cycle applied to the copper circuit plate are normally and regularly applied to the silicon nitride substrate. Due to these expansion and contraction of the copper circuit plate, a large stress is caused at the silicon nitride substrate whereby such stress has become a cause of forming the crack, allgatoring, crazing or the like.
In contrast, in aluminum, a plastic deformation caused by the heat cycle is liable to remain in the aluminum. For example, when the aluminum is applied with the heat cycle within a temperature range of 0xc2x0 C.-100xc2x0 C., the plastic deformation is gradually accumulated. Namely, the heat-expansion is accumulated, so that the aluminum is brought into a state where fine corrugations or wrinkles are formed in case of aluminum flat plate.
Accordingly, in case of the ceramic circuit board formed by bonding the aluminum circuit plate to the silicon nitride substrate, the aluminum circuit plate per se is deformed by the heat cycle while the stress due to the heat cycle is hardly caused to the silicon nitride substrate. Therefore, the cracks or allgatorings or the like hardly occur to the silicon nitride substrate.
Based on the above mechanism, in the ceramic circuit board according to fourth aspect of the present invention in which aluminum circuit plate is used as the metal circuit plate, it becomes possible to greatly improve an effect of suppressing the crack formation in comparison with a case where the copper circuit plate is used, the crack being caused at silicon nitride substrate when the circuit board is used under a condition where the heat cycle is applied.
In the ceramic circuit board according to fourth aspect of the present invention, in order to secure a high heat-radiating property of the ceramic circuit board, the silicon nitride substrate is required to have a thermal conductivity of 50 W/mK or more.
In addition, the ceramic circuit board according to fourth aspect of the present invention is characterized in that the silicon nitride substrate and the aluminum circuit plate are directly bonded without providing a brazing material layer therebetween. When Alxe2x80x94Si alloy plate containing Si is used as aluminum circuit plate, a more excellent bonding strength can be obtained. This means that Al brazing material layer used in the first and second aspects of the invention is replaced by Al plate or Al alloy plate, so that Alxe2x80x94Si alloy plate can exhibit effects as both a bonding layer and a circuit plate.
The present invention includes the above first to fourth inventions according to the respective aspects. The Si amount contained in the Al brazing material layer is preferably set to 7 wt % or less in the first invention. While, it is preferable to set the Si amount to 50 wt % or less in the second to fourth inventions.
In embodiments of the first invention, when the Si amount exceeds 7 wt % and, for example, the metal circuit plate is Cu plate, Cu plate and Al brazing material are melted and mixed thereby to be easily alloyed. When the alloying phenomena is advanced, a brittle intermetallic compound is disadvantageously formed thereby to lower the bonding strength.
In contrast, in embodiments of the second and third inventions, since the insulating layer for preventing the Al brazing material layer and the metal circuit plate from being alloyed is provided, there is no problem even if Si is added to the Al brazing material with an Si content-range of 50 wt % or less. Further, the metal circuit plate and the insulating layer slightly exhibit the alloying phenomena. However, when at least one element selected from Ib, IIb, IIIb and IVb family elements such as Sn, In or the like prescribed in periodic table is used as a component for constituting the insulating layer, the alloying phenomena can be limited to a range of 10% or less of the thickness of the metal circuit plate.
Similarly in the fourth invention, since Alxe2x80x94Si circuit plate is directly bonded, there is no need of considering the alloying of the metal circuit plate to be separately provided. Therefore, Si amount of 50 wt % or less can be applied. When Si is added at amount of exceeding 50 wt %, Si is solely crystallized thereby to lower the bonding strength. Accordingly, in the second to fourth inventions, the Si amount contained in the Al brazing material or Al circuit plate is set to 50 wt % or less, preferably set to 15 wt % or less.
Further, in the first to fourth inventions, the Si amount is preferably set to at least 0.01 wt %. this is because when the amount is less than 0.01 wt %, the effect by the addition cannot be obtained. In addition, to maintain the eutectic bonding, it is preferable to set the Si amount to 0.05 wt % or more.
Accordingly, when summing up the Si contents contained in the Al brazing material or the metal circuit plate in respective cases, in the first invention, the Si amount is set to 0.01-7 wt %.
In the second invention, the Si amount is set to 0.01-50 wt %. In the third invention, the Si amount is set to 0.01-50 wt %. In the fourth invention, the Si amount is set to 0.01-50 wt %.
The more preferable ranges of Si amounts are 0.05-1 wt % for the first invention, 0.05-15 wt % for the second invention, 0.05-15 wt % for the third invention, 0.05-15 wt %, 0.05-15 wt % for the fourth invention. In addition, in a case where the alkaline earth metal or the compound thereof is added, the amount is preferably set to a range of 0.01-12 wt % or less.
In addition, various ceramic substrates such as silicon nitride, aluminum nitride, aluminum oxide or the like can be applied as the ceramic substrate. However, when the silicon nitride substrate is used in embodiments of the first to fourth inventions, the Si amount to be contained in the brazing material can be reduced to 7 wt % or less, preferably to 1 wt % or less. This is because Si contributing to the bonding is formerly contained in the silicon nitride substrate, so that Si is supplied to the bonding layer from a side of the silicon nitride substrate.
Further, in the ceramic circuit board according to another aspect of the present invention, it is preferable that the ceramic circuit board has a multi-layered structure in which at least two aluminum circuit plates are laminated through the silicon nitride substrate.
According to this structure, due to the laminated structure of the ceramic substrates and metal circuit plates laminated in a thickness direction of the circuit board, a plain surface area required for installing the circuit board can be reduced, so that a mounting space with respect to electronic devices can be reduced. In other word, the laminated structure can contribute to advance the miniaturization of the electronic devices. In addition, even in such a case, the effect of suppressing the crack formation to be caused at the silicon nitride substrate by the heat cycle is not impaired at all on the basis of the aforementioned respective structures. Accordingly, there can be exhibited an excellent effect in view of both increase of strength and reduction of the mounting space.
FIG. 6 shows an another preferable example of a structure of a ceramic circuit board according to the present invention.
The ceramic circuit board 1c of the present invention is formed, for example, by bonding aluminum plate 3 as a metal circuit plate to a ceramic substrate 2. When an outer peripheral portion of the aluminum plate 3 is symbolized as 3a and an inside the outer peripheral portion 3a is worked to form a thinned portion 3b, it becomes possible to suppress to cause crack to the ceramic substrate during manufacturing and operating the ceramic circuit board. In this regard, the inside the outer peripheral portion denotes an area having a constant width ranging from the outer peripheral portion toward a center portion of Al plate.
For example, the aluminum plate is directly bonded to the ceramic plate, the plates are heated up to a temperature higher than melting point of aluminum plate. However, when the thinned portion is formed to the inside the outer peripheral portion of the aluminum plate, a thermal stress caused at the outer peripheral portion of the aluminum plate is absorbed by the plastic deformation of the thinned portion formed to the aluminum plate, so that it becomes possible to suppress the crack formation to be caused at the ceramic plate.
In addition, not only a time when the aluminum plate and the ceramic substrate are bonded but also a time when the bonded circuit board is operated after the bonding operation, the thermal stress is caused by the heat cycle. However, in also this case, the thermal stress caused at the outer peripheral portion of the aluminum plate is absorbed by the plastic deformation of the thinned portion formed to the aluminum plate, so that it becomes possible to suppress the crack formation to be caused at the ceramic plate.
A shape of the thinned portion 3b may be formed, for example, as shown in FIG. 7. This thinned portion 3b is formed by grinding a surface of the aluminum plate to which the ceramic substrate is not bonded, thereby to form the thinned portion as a step portion. As a method of forming the thinned portion 3b, working methods such as etching treatment or the like can be used other than the mechanical working method.
The thinned portion 3b may be formed after the ceramic substrate and the aluminum plate are bonded. However, when the thinned portion 3b is previously formed before the ceramic substrate and the aluminum plate are bonded, the crack formation to the ceramic plate to be caused by the thermal stress at the bonding operation can be suppressed, thus being more effective.
A forming range of the thinned portion 3b is as follows. For example, assuming that a length from the outer peripheral portion of the aluminum plate to the thinned portion 3b is indicated as W and a thickness of the thinned portion 3b is indicated as T, the length W is preferably set to 0.3-1.0 mm and the thickness T is preferably set to ⅙ to ⅚ of a thickness at a mounting surface of the aluminum plate. When the length W from the outer peripheral portion to the thinned portion is less than 0.3 mm, a sufficient stress-mitigating effect cannot be obtained, so that the crack formation is liable to occur. Further, when the length W exceeds 1.0 mm, the stress-mitigating effect will be insufficient and a mounting surface of the aluminum plate to which electronic parts such as semiconductor chip, terminal or the like is mounted will be disadvantageously decreased. When the thickness T of the thinned portion 3b is less than ⅙ of the aluminum plate at a mounting surface, there may be posed a fear that the strength of the aluminum plate is disadvantageously lowered. While, when the thickness T exceeds ⅚ of the aluminum plate at the mounting surface, the stress-mitigating effect cannot be observed.
Next, another embodiment of the ceramic circuit board 1d according to the present invention is shown in FIG. 8.
As shown in FIG. 8, there is provided the ceramic circuit board 1d according to the present invention in which the aluminum plate 3 as metal circuit plate is provided with a plurality of holes 6 formed at inside an outer peripheral portion of a surface opposing to a bonding surface of the aluminum plate to which the ceramic substrate 2 is bonded.
As a more preferable embodiment of the aforementioned ceramic circuit board 1d, it is preferable that, for example, the plurality of holes 6 are linearly arranged along the outer peripheral portion of the aluminum plate 3a. A cross-sectional shape of the hole 6 may be a circular-shape as shown in FIG. 9 or may be a rectangular-shape as shown in FIG. 10. These a plurality of holes can be formed by etching treatment, or can be also formed by a pressing work using a pressing die. When the plurality of holes are formed to be linearly arranged, it is possible to effectively mitigate a stress-concentration applied to the outer peripheral portion of the aluminum plate.
Regarding to a size of the hole, in a case where the cross-sectional shape of the hole is circle as shown in FIG. 9 and a diameter of the hole is denoted as D, the diameter D is preferably set to 0.3 mm to 1.0 mm. In addition, when an interval between the adjacent holes is denoted as L, the interval L is preferably set to 0.3 mm to 1.0 mm. Further, when a distance from the outer peripheral portion of the metal plate to the hole is denoted as Z, the distance is preferably set to 0.3 mm or more.
When the diameter of the hole is less than 0.3 mm, the stress cannot be sufficiently mitigated. While, when the diameter of the hole exceeds 1.0 mm, the strength of the aluminum plate is lowered and the mounting surface area is disadvantageously decreased. Further, when the distance between the adjacent holes is excessively large, there may be a fear that a sufficient stress-mitigating effect cannot be obtained. While, when the distance is excessively small, there may be a case to cause a deformation of the aluminum plate. Furthermore, when the distance Z from the outer peripheral portion of the metal plate to the hole is less than 0.3 mm, there may be a fear that a sufficient stress-mitigating effect cannot be obtained.
The aforementioned plurality of holes can be formed as non-penetrated holes 6a as shown in FIGS. 11 and 12, or can be formed as penetrated holes 6b as shown in FIG. 13. For example, the non-penetrated hole 6a can be easily formed by the pressing work or the like, thus being effective in saving manufacturing man-hour. In addition, when the metal circuit plate and the ceramic substrate are bonded by applying DBA method (active metal brazing method), the penetrated hole 6b also functions as an exhaust hole through which gaseous components such as oxygen or the like exhausted at without contributing to the bonding operation can be exhausted.
In this connection, in a case where the plurality of holes are formed to be non-penetrated holes 6a, a shape of the hole 6a in the depth direction may be formed so as to have an uniform shape as shown in FIG. 11, or may be formed so as to have an inverse circular cone shape as shown in FIG. 12.
Next, another embodiment of the ceramic circuit board according to the present invention will be explained hereunder.
As shown in FIG. 14, there is provided the ceramic circuit board 1e according to the present invention in which an aluminum plate 3 as metal circuit plate is provided with a plurality of non-continuous grooves 7 that are provided along an outer peripheral portion of the metal circuit plate and formed at inside an outer peripheral portion of a surface opposing to a bonding surface of the aluminum plate 3 to which the ceramic substrate 2 is bonded. When such non-continuous grooves are provided to the aluminum plate 3, the stress caused at the outer peripheral portion of the aluminum plate 3 can be effectively mitigated. In this regard, the inside the outer peripheral portion means a portion having a constant width ranging from the outer peripheral portion toward a center portion of the metal circuit plate. In the ceramic circuit board of this invention, the non-continuous grooves are preferably formed and arranged in a linear-shape. When the non-continuous grooves are formed so as to have a linear-shape, the stress-concentration can be mitigated more effectively.
FIG. 15 shows a partially enlarged view of a part of the ceramic circuit board 1e according to the present invention. In FIG. 15, the reference character Wxe2x80x2 denotes a width of a single groove, E denotes a length of the single groove, and Z denotes a distance from the outer peripheral portion to the groove.
A shape of each single groove constituting the noncontinuous grooves is as follows. Namely, it is preferable that the width Wxe2x80x2 of the single groove is set to 0.05-1.0 mm, the length E is set to 20 mm or less and the distance Z from the outer peripheral portion to the groove is set to 0.3 mm or more.
When the width Wxe2x80x2 is less than 0.05 mm, the stress cannot be sufficiently mitigated. While, when the width Wxe2x80x2 exceeds 1.0 mm, the strength of the aluminum plate is lowered and the mounting area is also decreased. Further, when the length E of each single groove exceeds 20 mm, an area to which the grooves are not formed is decreased, so that there may be a fear that the deformation of the aluminum plate cannot be suppressed. Furthermore, when the distance Z from the outer peripheral portion to the groove is less than 0.3 mm, there may be a fear that a sufficient stress-mitigating effect cannot be obtained.
In this connection, a cross sectional shape in a longitudinal direction of the non-continuous groove 7 may be formed to make a groove 7a having a U-shaped in cross section as shown in FIG. 16, or may be formed to make a groove 7b having an inverse triangular shape as shown in FIG. 17. The depth H of the groove 7 is preferably set to ⅙ to ⅚ of a thickness at a mounting surface of the aluminum plate.
When the depth H is less than ⅙ of the thickness at the mounting surface of the aluminum plate, there may be a fear that the effect of dispersing the stress would be insufficient. While, when the depth H exceeds ⅚ of the thickness at the mounting surface of the aluminum plate, the strength of the aluminum plate is liable to be lowered.
As the ceramic substrate to be used for the ceramic circuit board of the present invention, it is preferable to use a substrate mainly composed of at least one material selected from alumina, aluminum nitride and silicon nitride.